The present disclosure is generally directed to methods and apparatus for processing ceramic fiber. More particularly, the present disclosure is directed to methods and apparatus for processing ceramic fiber for the manufacture of ceramic matrix composite (CMC) articles.
Ceramic matrix composites (CMCs) generally comprise a ceramic fiber reinforcement material embedded in a ceramic matrix material. The reinforcement material may be discontinuous short fibers that are randomly dispersed in the matrix material or continuous fibers or fiber bundles oriented within the matrix material. The reinforcement material serves as the load-bearing constituent of the CMC in the event of a matrix crack. In turn, the ceramic matrix protects the reinforcement material, maintains the orientation of its fibers, and serves to dissipate loads to the reinforcement material. Silicon-based CMCs, such as silicon carbide (SiC) as the matrix and/or reinforcement material, have become of particular interest in high-temperature applications due to their high temperature capabilities, such as for use in components of gas turbines, including aircraft gas turbine engines and land-based gas turbine engines. SiC fibers have also been used as a reinforcement material for a variety of other ceramic matrix materials, including TiC, Si3N4, and Al2O3.
Continuous fiber reinforced ceramic composites (CFCC) are a particular type of CMC that offers light weight, high strength, and high stiffness for a variety of high temperature load-bearing applications, such as in shrouds, combustor liners, vanes (nozzles), blades (buckets), and other high-temperature components of gas turbines. A notable example of a CFCC material developed by the General Electric Company under the name HiPerComp® contains continuous SiC fibers in a matrix of SiC and elemental silicon or a silicon alloy.
Various techniques may be employed in the fabrication of CMCs, including chemical vapor infiltration (CVI), wet drum winding, lay-up, lamination, pyrolysis, and melt infiltration (MI). These fabrication techniques have been used in combination with tooling or dies to produce near-net-shape articles through processes that include the application of heat and chemical processes at various processing stages. Examples of such processes, particularly for SiC/Si—SiC (fiber/matrix) CFCC materials, are disclosed in U.S. Pat. Nos. 5,015,540, 5,330,854, 5,336,350, 5,628,938, 6,024,898, 6,258,737, 6,403,158, and 6,503,441, and U.S. Patent Application Publication No. 2004/0067316.
One process of manufacturing CMCs entails the use of CMC prepregs, which are typically sheet-like structures comprising the reinforcement fibers impregnated with a slurry that contains a precursor of the matrix material and one or more organic binders. The prepreg must undergo processing (e.g., firing) to convert the precursor to the desired ceramic matrix material. Prepregs for CFCC materials frequently comprise a two-dimensional fiber array comprising a single layer of aligned tows (bundles of individual filaments) impregnated with a matrix precursor to create a generally two-dimensional lamina. Multiple plies of the resulting prepregs are then stacked and debulked to form a laminate preform, a process referred to as “lay-up.” The prepregs are typically, but not necessarily, arranged so that tows of adjacent prepregs are oriented transverse (e.g., perpendicular) to each other, providing greater strength in the laminar plane of the preform (corresponding to the principal (load-bearing) directions of the final CMC article). As an example, FIG. 1 represents a surface region of a CMC article 10 including multiple laminae 12, each the result of individual prepreg tapes or sheets. As also shown in FIG. 1, each lamina 12 contains a ceramic reinforcement made up of unidirectionally-aligned fibers 17 encased in a ceramic matrix 14 formed by conversion of the ceramic matrix precursor (e.g., after firing).
As illustrated in FIG. 2, one process utilized in making prepreg CMC preforms includes a winding technique to form the fibers 20 (individual filaments or tows) into a unidirectional prepreg tape, which is then used for the lay-up of the composite preform. As represented in FIG. 2, some winding techniques involve coating the fibers 20. The fibers 20 are coated for several purposes, such as to protect them during composite processing, to modify fiber-matrix interface strength and to promote or prevent mechanical and/or chemical bonding of the fiber and matrix. A number of different techniques have been developed for applying coatings to ceramic fiber, such as slurry-dipping, sol-gel, sputtering and chemical vapor deposition (CVD). Of these techniques, CVD may be considered as being most successful in producing impervious coatings of uniform thickness and controlled composition. In a typical CVD process, the fibers and reactants are heated to an elevated temperature where coating precursors decompose and deposit as the coating.
Continuous fiber coating processes have been preferred for composites processed by the winding technique. In a continuous coating process, as shown in FIG. 2, fiber 20 is continuously passed through a CVD reactor 22 containing coating precursors 24 to form the coated fiber 26. As also shown in FIG. 2, a continuous fiber coating process may involve running a single fiber tow or filament 20 through the CVD reactor 22 at a time. The coating may be conducted at low pressure, and the fiber 20 may be transported through the reactor 22 at a slow speed, to insure uniform coating on the coated fiber 26. Such a CVD coating process suffers from a significant amount of broken fibers, and “loose” fibers when a fiber tow is coated (i.e., “fuzz”), which degrades throughput or yield of the process. Although such a fiber coating process may provide an effective coated fiber 26, there remains a need for further improvements to CVD coat fibers 20 with higher productivity.
As illustrated in FIG. 2, a winding technique may also form the coated fiber 26 (a filament or tow) into a unidirectional prepreg tape by impregnating the coated fiber 26 with a matrix precursor 27. For example, a wet drum winding processes for impregnating the coated ceramic fiber 26 may entail pulling the ceramic fiber 26 through a bath 27 of a matrix precursor slurry mixture that includes suitable matrix precursor materials, organic binders, and solvents, as shown in FIG. 2. The resulting precursor-impregnated fiber 28 is then wound around a drum 29 to form a planar unidirectional prepreg tape. Before contacting the drum 29, the precursor-impregnated fiber 28 is typically pulled through an orifice to control the amount of slurry picked up. By indexing the drum 29 (and/or the bath 27 and orifice), the precursor-impregnated fiber 28 is laid down at a constant pitch to yield a continuous, planar unidirectional prepreg tape. Prior to being wound with the precursor-impregnated fiber 28, the drum 29 may be wrapped with a release sheet so that the resulting prepreg tape can be more easily removed from the drum 29. While on the drum 29, the prepreg tape may be allowed to air dry by allowing the solvents to evaporate. Alternatively, the tape may be cut from the drum 29, laid flat, and allowed to air dry.
Prepreg tapes produced by such a wet drum winding processes may have a surface roughness, or waviness, corresponding to the pitch of the fiber 28 on the drum 29. There may also be variability in the distribution of fiber and matrix across the tape because of the pitch. Furthermore, because the fiber is under tension during the winding process, the impregnated fiber 28 may tend to be pulled down onto the drum surface, yielding a prepreg tape that has proportionally more fiber at the surface of the tape contacting the drum 29 and proportionally more matrix precursor at the surface of the tape facing away from the drum 29.
Such a wet drum winding process can also suffer from a significant amount of broken fibers, and loosely adhering fibers 20 (i.e., “fuzz”) when a tow is utilized, that can break off and cause blockage of the orifice. Consequently, drum winding operations may require constant operator supervision so that such blockages can be removed as they occur.
Another complication of a drum winding processes may revolve around necessity to completely impregnate (i.e., wet out) the fiber 20 with the slurry 27 during the winding process, which requires that the fiber 16 spend a sufficient amount of time submersed in the slurry 27. This submersion time, which can be about five seconds for certain processes, may place a limit on the speed with which the fiber 16 can be drawn through the slurry 27 bath. Consequently, the time necessary to drum wind a 100 meter fiber 20 tow can be relatively lengthy.
Accordingly, alternative methods and apparatus for coating and/or impregnating ceramic fiber (to form prepregs) for producing CMCs with improved yield or throughput are desirable.